MH2 is a member of the defective avian leukemia viruses carrying myc-related oncogenes. This group of viruses also includes MC29, CMII and OK10. Southern blotting of MH2-infected quail DNA revealed a 6.5 kb Eco RI fragment that hybridized to MC29 proviral DNA. This fragment was cloned from a lambda phage library constructed from a size-fractionated RI digest of MH2-infected quail DNA. DNA sequence analysis of the RI fragment indicated that it contained 99% of MH2 proviral genome 5.3 kb in length. Restriction enzyme and heteroduplex mapping showed that 3 feet to the gag-myc junction between MC29 and RSV, MH2 continued to carry a few hundred bases of gag sequence plus an additional 1 kb sequence not present in MC29 and RSV. The latter probably represented the 5 feet region of c-myc gene. Most of the gene coding for the envelope protein of MH2 has been replaced by the v-myc region found in MC29. The AMV transforming gene has also been inserted between the ClaI site and the BamHI site of the expression vector pJL6. The fusion protein was predicted to carry 13 amino acids specified by the lambda cII gene and the entire myb region except its first five amino acids to give a molecular weight of 32,000 daltons. When the lambda P1 promoter on the hybrid plasmid was derepressed, E. coli harboring the plasmid produced a high level of a 32K protein. This protein was not synthesized in E. coli grown under repressed condition.